For Roasted Red Pepper Ketchup: In a mini chopper or blender combined ketchup and roasted red peppers (reserve one red pepper to garnish the burgers). Blend until smooth. If the mixture is too thick and not blending add a tablespoon or two the liquid from the jar of roasted red peppers. Set aside
For Pesto Mayo: Combine mayonnaise and pesto in a small bowl and stir together. Set aside
For Burgers: Heat 3 tablespoons of canola in a large pan over medium low heat. Add onion and cook until golden brown and caramelized about 15 to 20 minutes making sure to stir every few minutes.
Form the ground beef into 12 equal sized mini patties. Heat the remaining canola oil in a large skillet over medium high heat. Season each side of each patty with the grill seasoning and place into the skillet and cook 3 minutes. Flip the burgers, top 4 with the blue cheese, the next 4 with 2 slices of cheddar cheese each and the final 4 with 2 slices of mozzarella cheese each. Cover with a lid and cook for another 2 to 3 minutes until cheese is melted.
Place the burgers onto the bottom half of each roll. Top the four blue cheese burgers with the caramelized onion then with the top half of the bun. Sliced the reserved red pepper into 4 pieces. Top the 4 cheddar burgers with the pieces of roasted red pepper, some of the roasted red pepper ketchup and the top half of the rolls. Finally top the four mozzarella burgers with the pesto mayo and the top half of the bun.